ci: ratchet the bundle budget and scan builds for credentials (FH-3.3, FH-3.5)

Bundle budget was warning-only: initial warning 700 kB, error 1.8 MB.
Measured today the initial bundle is 1.55 MB raw / 324.58 kB transfer -
up from the 1.15 MB measured on 11 August, so it had been growing with
nothing to stop it.

Lowers maximumError to 1.6 MB. That is a ratchet, not a target: just
above today's size so the bundle cannot grow, with the 700 kB warning
left in place as the goal. Lower it each time the number comes down.

Adds scripts/ci/scan-bundle.sh (npm run scan:bundle), run in CI after
the build. Seven patterns: both provider auth headers, the partner ID
shape, client_secret, private key blocks, AWS keys, Telegram bot
tokens. The legacy payment code that put credentials in the browser is
already deleted; this is what stops it coming back. Verified in both
directions - clean against the real dist, exit 1 against a planted
credential.

Measurement also corrected two assumptions recorded in the harvest
TODO: admin and editor code is already lazy-loaded, so the initial
bundle is main alone rather than a deployable-split problem; and mock
gateway fixtures do reach production chunks, which is now filed as
FH-E.6 with the cause identified.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/ci/scan-bundle.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# Fails the build if a production bundle contains anything that should only
+# ever exist server-side.
+#
+# Why this exists: the storefront used to send provider payment credentials
+# from the browser - an `authorization-key` header, a `userid-value` header,
+# and a hardcoded partner ID literal compiled into the bundle. That code is
+# gone (FH-1.3), and this check is what stops it coming back. A credential in
+# a JS bundle is not a leak you can revoke quietly; it is published.
+#
+# Usage:
+#   npm run build && scripts/ci/scan-bundle.sh [dist-dir]
+
+set -euo pipefail
+
+DIST="${1:-dist}"
+
+if [[ ! -d "$DIST" ]]; then
+  echo "scan-bundle: '$DIST' does not exist - build first" >&2
+  exit 2
+fi
+
+# Each entry is "label|extended-regex". Keep patterns specific: a pattern that
+# fires on ordinary code trains people to ignore this check.
+PATTERNS=(
+  "provider auth header|authorization-key"
+  "provider user header|userid-value"
+  "hardcoded partner id|web-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}"
+  "oauth client secret|client_secret[\"']?[[:space:]]*[:=]"
+  "private key block|BEGIN (RSA |EC |OPENSSH )?PRIVATE KEY"
+  "aws access key|AKIA[0-9A-Z]{16}"
+  "telegram bot token|[0-9]{8,10}:AA[0-9A-Za-z_-]{33}"
+)
+
+failed=0
+
+for entry in "${PATTERNS[@]}"; do
+  label="${entry%%|*}"
+  pattern="${entry#*|}"
+
+  if matches="$(grep -rIlE "$pattern" "$DIST" 2>/dev/null)"; then
+    if [[ -n "$matches" ]]; then
+      echo "FAIL: $label found in the built bundle" >&2
+      echo "$matches" | sed 's/^/       /' >&2
+      failed=1
+    fi
+  fi
+done
+
+if [[ $failed -ne 0 ]]; then
+  echo >&2
+  echo "A credential reached the browser bundle. Move it behind the API." >&2
+  exit 1
+fi
+
+echo "scan-bundle: clean ($DIST)"